This invention relates to a magnetic disk substrate made of a glass-ceramic having a crystal structure with improved surface characteristics after polishing, a magnetic disk formed by subjecting the magnetic disk substrate to a film forming process and a method for manufacturing the magnetic disk substrate.
There has been a rapid progress in developments in magnetic disks demand for which has recently increased remarkably as external media of large scale computers and personal computers. Characteristics required generally for a magnetic disk substrate are as follows:
(1) In CSS (contact start and stop) characteristics of a magnetic disk, stiction tends to occur between the head and the magnetic disk due to increase in contact resistance caused by high speed rotation of the magnetic disk, if the magnetic disk has a smooth surface with surface roughness (Ra) below 15 .ANG.. The magnetic disk therefore is required to have surface roughness of 15 .ANG. or more. Moreover, if the magnetic disk has an excessively rough surface having surface roughness exceeding 50 .ANG., damage to the head or the recording media tends to occur. The surface roughness of the magnetic disk substrate therefore should not exceed 50 .ANG.. PA0 (2) Since the amount of lifting of the head tends to decrease to the order of 0.1 .mu.m to 0.05 .mu.m owing to improvement of recording density of the magnetic disk, the surface of the disk must be flat and relatively smooth. PA0 (3) The material of the magnetic disk substrate must not include anisotropic crystal or other defects and its structure must be dense, homogeneous and fine. PA0 (4) The material must have sufficient mechanical strength and hardness for standing a high speed rotation and contact with the head. PA0 (5) The material must not contain Na.sub.2 O ingredient in principle because the Na.sub.2 O ingredient causes dispersion of sodium ion during the film forming process with resulting deterioration in characteristics of the film. PA0 (6) The material must have chemical durability against rinsing and etching with various chemicals. PA0 (1) Polishing is made after chemical tempering and so the tempered layer tends to cause instability in thinning the disk substrate. PA0 (2) For improving the CSS characteristics, the substrate must be subjected to mechanical or chemical processing for making texture. This prevents mass production of the product at a competitive cost. PA0 (3) Since the Na.sub.2 O ingredient is included as an essential ingredient in the glass, the film forming characteristic of the glass is deteriorated with the result that a surface coating processing becomes necessary. It has recently been put into practice in the field of chemically tempered glass and crystallized glass-ceramic to make the surface of the glass rough during the polishing process for overcoming the problem of the texture processing for improving the CSS characteristics. This technique however is not sufficient either for realizing a stable mass production of the product at a competitive cost.
Aluminum alloy has been conventionally used as a material of magnetic disk substrate. The alluminum alloy substrate, however, tends to produce a substrate surface having projections or spot-like projections and depressions during the polishing process due to defects inherent in the material. As a result, the alluminum alloy substrate is not sufficient in flatness and surface roughness and cannot cope with the recent requirement for high density recording necessitated by increase in the amount of information.
As a material for overcoming this problem of the alluminum alloy substrate, known in the art is a glass substrate for magnetic disks made of chemically tempered glass. This glass substrate, however, has the following disadvantages:
Aside from the alluminum alloy substrate and chemically tempered glass substrate, known also in the art are glass-ceramics which satisfy some of the above described requirements. For example, the SiO.sub.2 --Al.sub.2 O.sub.3 --Li.sub.2 O system crystallized glass-ceramic described in the Japanese Patent Application Laid-open No. Sho 60-229234 contains beta-quartz solid solution or beta-spodumene solid solution and has a grain diameter of crystal ranging from about 0.1 .mu.m to about 1.0 .mu.m. For another example, the SiO.sub.2 --Li.sub.2 O system glass-ceramic described in the Japanese Patent Application Laid-open No. Sho 62-72547 contains Li.sub.2 O.2SiO.sub.2 and Li.sub.2 O.SiO.sub.2 as the main crystalline phase. The Li.sub.2 O.2SiO.sub.2 phase has a rod-like crystal having a grain diameter of crystal in the range from about 0.3 .mu.m to about 1.5 .mu.m and the Li.sub.2 O.SiO.sub.2 phase has a granular crystal having a grain diameter of crystal in the range from about 0.3 .mu.m to about 0.5 .mu.m. U.S. Pat. No. 3,231,456 discloses a SiO.sub.2 --Li.sub.2 O--P.sub.2 O.sub.5 --MgO system glass-ceramic added with CuO and SnO ingredients in which Li.sub.2 O.2SiO.sub.2 is grown as a main crystal phase and alpha-quartz may be grown as a sub crystal phase. Since, however, these glass-ceramics require a heat treatment for crystallization in a high temperature range from 850.degree. C. to 1050.degree. C., they cannot grow a globular grain strucuture of aggregated particles as the grain structure of alpha-quartz as the glass-ceramic made according to the present invention as will be discussed in detail later. Besides, in U.S. Pat. No. 3,231,456, the crystal phase and crystal structure are discussed for providing a ceramic material which is suitable for use as a bonding material.
U.S. Pat. No. 3,977,857 discloses a glass-ceramic of a SiO.sub.2 --Li.sub.2 O--MgO--P.sub.2 O.sub.5 --(Na.sub.2 O+K.sub.2 O) system which, as a metal bonding glass-ceramic, is suitable for use as an electric insulating material which can be bonded directly to a metal member. The specification of this U.S. patent teaches that the main crystal phase of the glass-ceramic obtained is Li.sub.2 O.2SiO.sub.2 and does not teach anything about the globular grain structure of aggregated particles as the grain structure of alpha-quartz as in the present invention.
Japanese Patent Application Laid-open No. Sho 63-210039 discloses a glass-ceramic of a SiO.sub.2 --Li.sub.2 O--MgO--P.sub.2 O.sub.5 system which is suitable for use as a magnetic head substrate. The specification of this publication teaches that the main crystal phases of the glass-ceramic obtained are Li.sub.2 O.2SiO.sub.2 and alpha-cristobalite and does not teach anything about the globular grain structure of aggregated particles as the grain structure of alpha-quartz as in the present invention. Besides, the nucleating temperature is 550.degree. C.-800.degree. C. and the MgO ingredient is added for the purpose of restricting variation in a coefficient of thermal expansion by coexistence with the P.sub.2 O.sub.5 ingredient.
In short, crystal phases growing in the prior art Li.sub.2 O--SiO.sub.2 system glass-ceramics (i.e., Japanese Patent Application Laid-open No. Sho 62-72547, U.S. Pat. Nos. 3,231,456, 3,977,857 and Japanese Patent Application Laid-open No. 63-210039) are Li.sub.2 O.2SiO.sub.2 which constitutes the main crystal phase and a small amount of SiO.sub.2 (alpha-cristobalite or alpha-quartz) which constitutes the sub crystal phase. Therefore, in these prior art glass-ceramics, the Li.sub.2 O.2SiO.sub.2 crystal phase plays a major role and the sub crystal phase of alpha-quartz or alpha-cristobalite does not. These prior art glass-ceramics cannot provide the surface roughness of 15 .ANG. to 50 .ANG. which is necessary for the CSS characteristics of the magnetic disk as the surface roughness inherent in the glass-ceramics after polishing. These prior art glass-ceramics require some kind of texture processing for making the surface of the glass-ceramics rough after the polishing process for improving the CSS characteristics which are essential for the material of a magnetic disk substrate.
It is, therefore, an object of the invention to provide a magnetic disk substrates made of a glass-ceramic which has eliminated the disadvantages of the prior art products and has excellent surface characteristics after polishing by controlling the crystal structure and crystal grain of a crystal produced. It is also an object of the invention to provide a magnetic disk made by subjecting this magnetic disk substrate to a film forming processing. It is also an object of the invention to provide a method for manufacturing such a magnetic disk substrate.